In a cross ram intake manifold, as commonly used in electronic fuel injected engines, the manifold runners extend transversely of the longitudinal dimension of the manifold. As used with an eight cylinder engine, a first group of four runners extend diagonally down from one side of the manifold toward the opposite side, while a second group of four runners extend diagonally down from the opposite side of the manifold. Thus, both groups of runners extend transversely of the engine in a crossing pattern.
The lower end of each runner communicates with a cylinder of the engine, while the upper end of each runner communicates with the interior of an air plenum. Fuel is fed into each runner through a fuel injector which is mounted within an opening in the runner intermediate the ends thereof. Air is introduced into the plenum through a throttle body and is then drawn into the open upper ends of the runners.
It is known that varying the runner length and cross section will effect the tuning of the runner. If the length of the runner is increased, a lower RPM peak will be obtained. However, by increasing the runner length, the overall size of the manifold will normally also be increased. Theoretically, as the RPM peak is increased, the cross-sectional area of the runner should increase and the runner length should decrease. However, in practice, the configuration of the runners is determined primarily by packaging or size constraints.
With a marine engine, it is desirable to tune the engine to obtain high torque at low RPM for the high torque is desirable for pulling water skiers and for getting the boat on a plane.
The typical plenum, as used in a marine engine, is an aluminum casting and is bolted from the bottom to the manifold. The typical plenum includes an upper wall and a lower wall, which is spaced from the upper wall to provide an air chamber therebetween. If top bolting is employed, in which the heads of the bolts are located on the upper surface of the plenum, torquing down of the bolts can cause deformation of the upper wall of the plenum. As a consequence, it has been customary to use bottom bolting for connecting the plenum to the manifold, in which the heads of the bolts bear against the manifold and the bolts extend upwardly and are threaded into the plenum. However, With bottom bolting the bolt heads are located in relatively inaccessible locations which makes servicing difficult.